This invention relates to the manufacture and processing of liquid paraformaldehyde, by which is meant liquid mixtures consisting essentially of water and paraformaldehyde with the paraformaldehyde being present in concentrations ranging from approximately 70% up to about 91% by weight. More particularly, the invention is directed especially to the manufacture and handling of liquid paraformaldehyde containing approximately 80% to approximately 89% paraformaldehyde.
Liquid paraformaldehyde has two major industrial uses. First, it is an intermediate in preparing the relatively water-free hemiformal compositions which are available from, for example, the Celanese Chemical Company under the tradename "Formcel". Second, liquid paraformaldehyde is an intermediate in the manufacture of solid flake or powder paraformaldehyde, which is prepared by concentrating aqueous formaldehyde solutions by vacuum evaporation followed by cooling and flaking the resulting concentrate. The flake product can be subsequently ground to a powder if desired. The manufacture of flake paraformaldehyde in this manner, including the step of concentrating a relatively dilute formaldehyde solution to the liquid paraformaldehyde product with which the U.S. Pat. Nos. 2,568,016 and 2,568,017 to MacLean and Heinz, assigned to Celanese Corporation of America. The heart of the process as described in these patents is the vacuum evaporation of, for example, 40% to 50% aqueous formaldehyde under controlled conditions of temperature, pressure, pH and liquid retention time in a two-stage vacuum evaporation system. Liquid paraformaldehyde as contemplated by the present invention can be drawn from either of the two evaporation stages, the liquid drawn from the first evaporation stage being, of course, more dilute than that from the second stage.
The details of the prior-art concentration process just summarized above are not relevant to the present invention except that, regardless of whichever of the myriad possible modifications of this evaporation process one might happen to be employing, it will be characterized by the presence of concentrated liquid paraformaldehyde which, in turn, will be in contact with the interior surfaces of the processing equipment including vessels, piping, and, of particular importance in the present context, heat exchangers.
The present invention is directed to a processing problem which has been found to affect all process apparatus containing concentrated liquid paraformaldehyde but especially heat exchangers, such as the heating elements of the evaporators which are used in evaporating formaldehyde to produce the concentrate. Briefly, this problem is that liquid paraformaldehyde, especially when it is being heated, tends to deposit solid paraformaldehyde of high molecular weight, with resulting fouling of the surfaces of the process equipment, especially heat exchangers, such that it becomes necessary to interrupt operations for the purpose of removing these deposits. These interruptions in the manufacturing process are, of course, costly and troublesome.
It might be expected from the existing prior art that the apparatus fouling problem could be alleviated by incorporating into the paraformaldehyde solutions which are being processed any of a large number of stabilizers which are known to be effective in maintaining the clarity of those aqueous solutions containing 50% or so of formaldehyde which are the usual form in which formaldehyde is stored and transported. (It will be understood that the concentrated paraformaldehyde solutions to which the present invention is directed are normally used quickly at or near the site where they are produced. They are not stored for any length of time nor are they shipped for any great distance.) The prior-art formaldehyde stabilizers are, broadly, organic colloids ranging from naturally-occurring gums to synthetic high-molecular weight materials such as cellulose esters and polyvinyl alcohol. The prior art as related to known stabilizers for aqueous formaldehyde solutions is exemplified by U.S. Pat. Nos. 3,137,736; 3,379,769; 3,532,756; 3,745,190; 3,781,365; and 3,816,539.
Surprisingly, however, it has been discovered that incorporating stabilizers of the type taught in the prior art, which are generally organic colloids or in any event molecules of high molecular weight, has no significant effect in alleviating the problems discussed hereinabove which are encountered in processing liquid paraformaldehyde. Thus, the known prior art affords no assistance in coping with the problem, and those engaged in manufacturing and/or handling concentrated liquid paraformaldehyde have continued to seek a method for alleviating the problem of apparatus fouling and the resultant interruptions in plant operation which stem from it.
It is an object of the present invention to provide a method for retarding the deposition of solid paraformaldehyde from liquid paraformaldehyde compositions. It is another object to provide a method for extending the onstream time of apparatus which is employed in manufacturing and/or utilizing liquid paraformaldehyde. It is another object to provide new compositions of matter, in the form of concentrated liquid paraformaldehydes which have a reduced tendency to deposit solid paraformaldehyde on the interior surfaces of process apparatus within which they are contained. Other objects will be apparent from the following detailed description and claims.